Alloys based on precious metals are of special importance for their catalytic and magnetic properties useful in information storage media, magnetic refrigeration, audio reproduction and magnetic sealing. Transition metals such as palladium, platinum and cobalt are well known for their catalytic capabilities in bulk and deposited phases. Such alloys can be fabricated through bulk metal processes or through nanoparticle synthesis.
Presently available methods for synthesis of bulk metal alloys include physical methods such as mechanical deformation, thermalization of amorphous alloys, inert gas evaporation, and sputtering; and chemical methods such as reduction using NaBH4, NaBEt3H, alkali or alkaline earth metals, alcohol, sonichemical synthesis (Suslick and Price, Annu. Rev. Mater. Sci., 1999, 29:295-326), thermal decomposition and electrochemical methods.
These methods are complicated and demanding, typically involving sophisticated instrumentation. They also suffer from disadvantages such as contamination from mechanical parts. Although the chemical methods offer advantages over physical methods, such as chemical homogeneity at the molecular level, these methods also have disadvantages such as contamination from reaction byproducts, agglomeration and difficulty in scalability. (K. E. Gonsalves et al., Chemical Synthesis of Nanostructured Metals, Metal Alloys and Semiconductors, in Handbook of Nanostructured Materials and Nanotechnology, Vol. 1, Synthesis and Processing, 1-55 (H. S. Nalwa ed., Academic Press, 2000)).
An alternate method of synthesizing bulk metal alloys has been disclosed. This method involves the formation of a cyanogel through the reaction of a tetrachlorometallate with a transition metal cyanometallate complex in an aqueous environment (Heibel et al., Chem. Mat., 1996, 8:1504).
Presently, methods of synthesis known for synthesizing metallic nanoparticles include mechanical methods, such as grinding large particles, and chemical reduction in which a reducing agent, such as sodium borohydride, is used to reduce a dissolved metal ion species to a metallic particle. The latter approach usually involves the introduction of a surface-protecting agent into the solution so that the formed particles do not agglomerate. Both of these techniques are satisfactory for the production of metal nanoparticles of a single metal; however, they have serious shortcomings if an alloy nanoparticle, i.e., a nanoparticle of a homogenous solution of two or more metals, is the desired product. The differences in reaction rates and mechanical properties of the metals, due for example to the different redox potentials of the metals, limit the ability of the metals to form true alloy particles. Typically, when a mild reducing agent is employed in an effort to make alloy nanoparticles, a mixture of single-metal particles is obtained, rather than metal alloy particles. Even in cases where the desired alloy does form, the proportions of the metals in the particles vary widely. Further, the presence of trace surface-absorbed organics on the metallic particles from the use of surface-protecting agents significantly impairs the catalytic properties of the metallic particles.
Alloy metal particles in the nanometer range are key components of materials such as heterogeneous chemical catalysts and magnetic recording media (tapes and disk drives). Thus, a method of reproducibly synthesizing well-defined, non-agglommerated metal alloy particles of controlled size and composition are highly desired and as yet not readily available.